Method for Producing a Semiconductor Component with Two Trenches

ABSTRACT

A method, in which a first isolating trench, filled with a dielectric material, and a second conducting trench, filled with an electrically conductive material, can be produced. To this end, the first and second trenches are etched with different trench widths, so that the first trench is filled completely with the dielectric material after a deposition of a dielectric layer over the entire surface with the edges covered, whereas the wider second trench is covered by the dielectric layer only on the inside walls. By anisotropic back-etching of the dielectric layer, the semiconductor substrate is exposed at the bottom of the second trench. Subsequently, the second trench is filled with an electrically conductive material and then represents a low-ohmic connection from the substrate surface to the buried structure located below the second trench.

In semiconductor elements, it is possible to use buried, doped layers soas to connect vertically oriented components from “below.” It is alsopossible to provide a buried layer for isolation or shielding purposes.In all cases, such a buried layer needs a contact with the surface,which is undertaken via a low-ohmic doping that extends to the buriedlayer.

Additionally to this, isolating trenches are needed in semiconductorelements, so as to isolate component structures from one another. Thus,for example, there is a need to isolate a buried layer that iscorrelated with a first component structure from a second semiconductorstructure that is not in direct electrical connection with it; this mayalso be, for example, a buried layer. Such an isolating trench thenrequires a depth that surpasses that of the buried layer, so as tocreate a secure isolation with a high breakdown voltage.

From published US Patent Application US 2004/0018704A1, a method isknown, with which a low-ohmic connection of one component surface can beproduced from a component surface to a buried layer and simultaneously,an electric isolation between two sections of a buried layer. To thisend, a trench to the buried layer is first etched and then a dopingsubstance is introduced into the trench walls; this substance thencreates low-ohmic contact structures there. In the following step, thetrench is further etched to a desired depth required for the isolation,and the inside walls are finally coated with a dielectric.

Furthermore, the electric contacting of buried layers via so-calledsinker dopings is known, which are created at a certain depth of thesemiconductor substrate and by diffusion produce the low-ohmic zone forthe contacting of the buried layer with the surface.

The contacting of a buried layer via a sinker contact has thedisadvantage that while driving the doping to the desired depth,simultaneously a lateral diffusion takes place and the lateral expansionof the sinker contact is thus unnecessarily increased and anunnecessarily large amount of surface is taken up, which can no longerbe used for other component structures.

The object of the present invention is therefore to indicate a methodfor the production of such a contact structure, which is compatible withother process steps used for the production of the component andtherefore, the parallel production of other semiconductor structures ofthe semiconductor element is made possible.

This object is achieved according to the invention by a method with thefeatures of claim 1. Further embodiments of the invention derive fromthe other claims.

A method is indicated that makes possible the combined production in acommon semiconductor substrate of a first trench filled with adielectric and a second trench filled with a conductive material. Tothis end, the two trenches are first etched in the semiconductorsubstrate, wherein the second trench is produced with a larger widththan the first trench. Subsequently, a dielectric is deposited on theentire surface with the edges covered, in such a thickness that thefirst trench is completely filled with the dielectric; the second trenchwith the larger width, however, still remains partially opened. Then thedielectric is anisotropically etched until the substrate is exposed onthe bottom in the second trench, whereas the side walls remain coveredby the oxide layer. Such an etching step is also known as spaceretching.

The production of both trenches is simplified with the simultaneousproduction of isolating and contacting trenches. The distinction betweencontacting and isolating trenches takes place on the basis of theprocess conditions alone through the corresponding selection of thetrench width, which can be specified by lithography or also byself-adjusting structures.

It is advantageous to produce the first (isolating) trench with a depthd4 and the second (contacting) trench with a depth d5, wherein d4 isgreater than d5 and thus the isolating trench extends to a greater depththan the contacting trench. In this manner, it is possible by means ofthe first trench to isolate the buried structure, which is contactedwith the second trench, with respect to adjacent structure elements inthe semiconductor element.

Advantageously, the depth d4 of the first trench is selected so largethat the trench extends at least under the lower edge of the buriedstructure or the buried layer. The contacting second trench isadvantageously produced so deep that it reaches the buried structure,but in no way does it completely go through it. It is also possible toproduce the second trench at a depth d5 that is less than the upper edgeof the buried structure and to produce the contact to the buriedstructure only by diffusion of a doping substance that is introducedinto the trench via the conductive material.

The different depth of the two trenches can be attained with a two-partetching process, in which the first trench is first etchedanisotropically to a first depth d3 in a first partial etching step.Subsequently, in a second partial etching step, the first trench isetched further to its final depth d4 and simultaneously, in the samestep, the second trench is etched anisotropically to a depth d5. Thedepth of the first trench is thereby produced from the total depth ofthe trench structures produced in the two partial etching steps. Thedepth of the second trench is attained alone in the second partialetching step. The second partial etching step is therefore preferablycarried out in such a way that the desired depth d5 of the second trenchis attained.

The depth d3 of the first trench attained in the first partial etchingcorresponds thereby to the depth difference between the first and secondtrenches, and is selected in such a manner that the desired isolation orthe desired breakdown voltage is attained between the componentstructures to be isolated by this trench.

The division of the trench etching process into two partial steps isdone simply via a hard mask, which has first and second openings for thefirst and second trench. In the first opening, the substrate surface,that is, the semiconductor surface, is exposed. In the second opening,the thickness of the hard mask is reduced to a layer thickness d2; theother areas of the hard mask have the original layer thickness d1, withd2<d1.

With this hard mask, which has openings at different depths, thetrenches are subsequently produced in an anisotropic etching process,wherein the depth difference of the two trenches is dependent on theresidual thickness d2 of the hard mask in the second opening and theselectivity of the etching process or its etching rate ratio in theetching of the hard mask and the semiconductor material. The etchingprocess is adjusted in such a manner that the etching rate of the hardmask is substantially smaller than the etching rate of the semiconductormaterial. Via a suitable dimensioning of the residual layer thickness d2of the hard mask, therefore, it is possible to adjust the depthdifference between the first and second trenches with a givenselectivity of the etching process. The depth d3 corresponds thereby tothe product from the residual thickness d2 and the corresponding etchingrate ratio. With a given residual layer thickness d2 of the hard mask,one can of course also adjust the selectivity of the etching process,which, however, is expensive as a rule and not preferred.

The first and second trenches with different depths can also be producedin a second method variant, in which before the trench etching, a firstand, above it, a second resist layer are produced and subsequentlystructured. In the first resist layer, only the openings are therebyprovided for the first trench. In the second resist layer, openings forthe first and second trenches are produced.

In a first etching step, the substrate is etched anisotropically andselectively with respect to the first and second resist layers, whereinthe substrate, which is exposed only in the openings of the first resistlayer, is etched in the area of the first trench to a first freelyselectable depth d7. In the following step, the first resist layer isselectively etched with respect to the second resist layer, so as toalso expose the semiconductor substrate in the area of the secondopenings in the first resist layer.

In the next step, the substrate is anisotropically and selectivelyetched with respect to the first resist layer, wherein the first trenchis etched to its second and thus final depth d4 and the second trench toa depth d5. Before this step, it is possible to remove the second resistlayer. All partial steps can be carried out without interruption in thesame etching reactor, wherein the selectivity of the individual etchingsteps can be adjusted by a selection of corresponding etching conditionsin the etching reactor, such as gas composition, pressure, and/ortemperature.

For these methods, photoresist layers can be used as the first andsecond resist layers. For this, it is advantageous to use, as ananisotropic etching process, a chemically dominated plasma etchingprocess, which includes ions that are reactive with respect to thematerial of the layer to be etched.

The anisotropy of this method can be enhanced in that, in the meantime,the plasma conditions are so varied that the deposition ratepredominates, in comparison with the etching rate, and in this way, apassivation layer is deposited on all surfaces. In connection with theanisotropic etching process, the side wall above all is thus passivatedand no longer attacked. A halogen-containing plasma, in particular afluorine-containing plasma, is suitable for the etching process. Thealternation between the etching conditions and the deposition conditionscan be alternatingly carried out several times. This method can also beused to produce the openings in the first and/or second resist layers.

For this method, it is advantageous to cover the semiconductor substratewith a dielectric double layer consisting of an oxide layer and anitride layer. This can be used as an etching stop, in a later methodstep. With the double layer, it is necessary to have another etchingstep, with which openings are created in this double layer, precedingthe process. The width of the openings is selected larger in the doublelayer than the corresponding first and second openings for the first andsecond trenches in the first resist layer. This ensures that anunderetching of the first resist layer that may appear does not lead toan underetching of the double layer. Accordingly, the mask layout, thatis, the difference between the structure widths in the first resistlayer relative to the structure widths of the openings in the doublelayer, is selected in accordance with the extent of underetching to beexpected.

An underetching of the double layer is disadvantageous for thecomponent.

In both method variants that are to be distinguished in principle, adielectric, in particular a high-temperature oxide, is deposited afterthe etching of the two trenches to their final depth, which can bedeposited with a good edge covering even at the bottom of trencheshaving a high aspect ratio.

Doped polysilicon, tungsten silicide, or any other conductive,trench-filling, depositable material is suitable as a conductivematerial for the filling of the contact trench (second trench). To thisend also, a process is required that covers edges and also allowsdeposition on the bottom of a deep, second trench in such a way that thetrench is completely filled without the formation of hollow spaces.

For the removal of the deposited conductive material, which is not usedas trench filling, a back-etching or a planarizing, for example CMP(chemical mechanical polishing), can be used.

The back-etching is carried out until conductive material not depositedin the trench is completely removed from the surface.

Below, the invention is explained in more detail in conjunction withembodiments and corresponding figures. These are drawn in a purelyschematical manner and not true to scale.

FIG. 1 shows a component after the filling of the trenches;

FIGS. 2 to 12 show various method stages of a first embodiment;

FIGS. 13 to 22 show various method stages of a second embodiment.

FIG. 1 shows in a schematic cross-section a component structure by wayof example, as it can be produced with the proposed method. Thiscomprises a semiconductor substrate SU, in which a first trench G1 and asecond trench G2 are produced at a distance from one another. The depthof the first trench is greater than the depth of the second trench.Whereas the first trench is filled with a dielectric, the second trenchG2 is isolated on its side walls, relative to the substrate, and has afilling with a conductive material, which, on its lower end, contacts aburied structure VS, for example a buried layer. The surface of thesubstrate SU can be covered with an oxide layer OS.

In a first embodiment, one begins with a semiconductor substrate SU, inwhich a buried structure VS is provided at a distance to the surface.The buried structure VS can, for example, be produced in the surface ofa wafer and covered with an epitaxial layer. A dielectric layercombination SK of thin dielectric layers, which may be an oxide layerand a nitride layer, for example, is located on the semiconductorsubstrate (FIG. 2); they can be used as protective layers and etchingstop layers. A hard mask layer HS, an oxide layer, for example, isproduced over this, covering the entire surface. In the first step, amask opening of the hard mask is then produced for the first trench, inthat a correspondingly structured first resist mask RM1 is produced,which has a resist opening R01, as shown in FIG. 2, in the area of thefirst trench.

In the next step, the structure of the first resist mask is transferredto the hard mask layer HS by means of an anisotropic etching process.The dielectric layer combination SK can be used as an etching stop.Subsequently, the dielectric layer combination SK on the bottom of thetrench can be removed. FIG. 3 shows the arrangement with the first hardmask opening HM01, which has a width w1 and a depth d1.

In the next step, the hard mask opening for the second trench isproduced, in that a second resist mask RM2 is placed and correspondinglystructured (FIG. 4). The second resist opening R02 in the area of thesecond trench is transferred to the hard mask layer HS by means of ananisotropic etching process. The etching is carried out, while beingcontrolled by its duration, for example, in such a manner that thesecond mask opening HM02 is produced only to a depth d2<d1 and aresidual layer thickness of the hard mask remains on the bottom of thesecond hard mask opening. FIG. 5 shows the arrangement after theproduction of the hard mask opening and FIG. 6 after the removal of thesecond resist mask RM2. There, the finished hard mask HM is also shownwith first and second hard mask openings HM01, HM02. The second hardmask opening has a width w2, which is larger than the width w1 of thefirst hard mask opening.

In the next step, a first partial etching into the semiconductorsubstrate is carried out by means of an anisotropic etching process, aphysically dominated plasma etching process, for example. In the area ofthe first mask opening HM01, a first partial trench G1 a with a depth d3is thereby produced. As a result of the not one-hundred-percentselectivity of the etching process used, the hard mask layer is erodedin the area of the second hard mask opening HM02, until either the layercombination SK used as the etching stop layer or the surface of thesubstrate SU is exposed. FIG. 7 shows the arrangement at this processstage.

In the next step, a second partial etching process into thesemiconductor substrate SU is carried out, wherein the first trench G1is etched to its final depth d5 and the second trench G2 to a depth d4.FIG. 8 shows the arrangement at this process stage.

In the next step, optionally a channel stop doping is carried out in thetrench walls and, in particular, in the trench bottom. The purpose ofthis is to inhibit the generation of an inversion layer along the insidewalls of the trench and thus to increase the threshold voltage of thegeneration of parasitic conductive areas. The doping of the substrate ispreferably increased thereby.

In the next step, an edge-covering, trench-filling, dielectric layer DS,a high-temperature oxide, for example, is deposited on the entiresurface. This is produced in a layer thickness that corresponds to atleast half the width (w1)/2 of the first trench and therefore leads tothe complete filling of the first trench with the dielectric layer. Inthe area of the second trench, the dielectric layer produces only acovering of the trench walls and the trench bottom, on which it isdeposited in a layer thickness d6. FIG. 9 shows the arrangement at thisprocess stage.

In the next step, the dielectric layer DS is etched back in ananisotropic etching process, similar to a spacer etching, until thedielectric layer DS is completely removed at the bottom of the secondtrench G2. The dielectric layer combination serves as an etching stoplayer on the surface of the semiconductor substrate. The side walls ofthe second trench G2 remain covered by the dielectric layer DS;likewise, the first trench remains filled with the dielectric material.FIG. 10 shows the arrangement after the removal of the silicon nitridelayer, which forms the topmost layer of the dielectric layercombination.

In the next step, the second trench is filled with a conductivematerial, in that a conductive material is deposited, covering theedges, in a layer thickness that corresponds at least to half the trenchwidth (w2)/2 of the first trench. FIG. 11 schematically shows thearrangement at this process stage.

In the next stage, the conductive layer LS is anisotropically etchedback so that the conductive material LM remains exclusively in the areaof the first trench, as the trench filling; in the rest of the surfacearea, however, the lower partial layer of the dielectric layercombination, usually an oxide layer, remains.

FIG. 12 shows the arrangement at this process stage, which correspondsto the structure shown in FIG. 1.

Below, the production of the trench structure is described in moredetail in accordance with a second embodiment. One begins, as before,with a substrate SU with a buried structure VS, having a surface that iscovered by a dielectric layer combination SK. Appropriate openings areetched in the layer combination SK in the area of the first and secondtrenches, by means of a third resist mask RM3. FIG. 13 shows thearrangement at this process stage.

In the next step, the third resist mask RM3 is removed and a fourthresist mask RM4 is placed and structured. To this end, an opening,having a width that is smaller than the width of the opening produced inthe dielectric layer combination, is produced in the area of the secondtrench. The fourth resist mask RM4 remains unstructured in the area ofthe later second trench (FIG. 14).

In the next step, the structured, fourth resist mask is hardened in itsstructure, which can take place, for example, by treatment with UVradiation and by a tempering step, depending on the resist materialused. In the next step, then, a fifth resist mask RM5 is produced inthat a resist layer is placed and correspondingly structured. In thefifth resist mask RM5, openings are produced in the area of the firstand second trenches. The width w5 of the first resist opening R051(opening for the first trench in the fifth resist mask) is larger thanthe width w4 of the corresponding opening in the fourth resist mask RM4lying underneath. The fifth resist mask RM5 can be structured similar tothe third resist mask in FIG. 13. Correspondingly, the opening R051produced therein can be aligned with the edges of the openings in thedielectric layer combination SK. FIG. 15 shows the arrangement at thisprocess stage.

In the next step, a first partial trench is etched in the siliconsubstrate to a depth d7, using an anisotropic etching process, whichetches the semiconductor material of the substrate selectively to thematerial of the fourth and fifth resist masks RM4, RM5. To this end, achemically dominated plasma etching process is used, which is adjustedby varying the plasma conditions in at least one time section in such away that a material deposition and, in particular, the deposition of apassivation on the trench walls takes place, which increases theselectivity and the anisotropy of the process. FIG. 16 shows thearrangement after the creation of the first partial trench of a depthd7.

In the next step, the structure of the fifth resist mask RM5 istransferred to the fourth resist mask, wherein in the correspondingopenings, the material of the fourth resist mask is removed. Thestructure shown in FIG. 17 thereby arises; it now also has a secondresist opening R042, in the area of the second trench, in which thesurface of the substrate is exposed.

In the next step, in turn, the semiconductor substrate is etchedselectively to the fourth resist mask RM4, wherein the same etchingconditions as in the first partial etching step can be established. Thefirst trench is deepened to a final depth d8, whereas the second trenchis etched to a depth d9. FIG. 18 shows the arrangement at this processstep.

In the next step, an edge-covering dielectric layer DS is deposited overthe entire area in a layer thickness that is suitable for the completefilling of the first trench and which, in the second trench, covers theside walls and the bottom, but leaves a free space in the middle. FIG.19 shows the arrangement at this process stage.

In the next step, the further procedure can be as described inconjunction with FIGS. 10 to 12 for the first embodiment. An anisotropicspacer etching process follows, in which the layer thickness of thedielectric layer DS is removed until the surface of the semiconductorsubstrate is exposed on the bottom of the second trench. The dielectriclayer combination SK can be used as an etching stop in the area of theremaining surface. FIG. 20 shows the arrangement at this process step.

In the next step, a conductive material LS is deposited over the entirearea, in a manner covering the edges and filling the trenches, until thesecond trench is completely filled. FIG. 21 shows the arrangement atthis process stage.

In the next step, the layer of the conductive material is etched back,or the arrangement is planarized, until all of the conductive materialoutside the second trench is removed. FIG. 22 shows the arrangement atthis process stage, which corresponds to the possible final structureshown in FIG. 1.

The advantage of this second variant is to be found in that the etchingprocess can be carried out substantially faster than in the firstvariant.

The structure of the fourth and fifth resist masks is selected in such amanner as to compensate for an erosion of the fourth and fifth resistmasks as well as to provide, afterwards, still a sufficient reserveprojecting over the edges of the dielectric layer combination, so as tocompensate for an underetching of the fourth resist mask, which startsat the beginning of the etching process, and thereby, however, to avoidan underetching under the dielectric layer combination SK, as can beseen, for example, with the aid of FIG. 16.

At the beginning of the second partial etching step, as, for example,shown in FIG. 18, underetching no longer takes place because of thepassivation within the first trench. The underetching in the area of thesecond trench is also compensated by a corresponding structure layout ofthe fifth resist mask there, so that the underetching does here notreach below the dielectric layer combination either. As a result of thematerials used and the increased etching speed, the second processvariant can furthermore be carried out at a lower cost.

The result, however, is that both variants lead to the simultaneousproduction of the first and second trenches at different depths, whereinthe difference in depth can be adjusted in the desired manner. Thus, thefirst trench completely filled with dielectric can be selected so deepthat it comes to lie below the deepest electrically conductive structure(here: buried structure VS) of the semiconductor component and thereforereliably isolates it from neighboring component areas with electricallyconductive component structures. Also, the depth of the second trenchcan be established in a controlled manner with the second partialetching step, so that in the first trench, it is precisely the upperedge of the buried structure VS that is exposed. The electricallyconductive material LM deposited in the trenches makes the second trencha low-ohmic connection to the buried structure, which can thus becontacted electrically via the conductive material in the second trench.

The invention is not limited to the embodiments that are explained andrepresented in the figures. Rather, the two trenches can be producedwith other mask combinations and other etching processes. A large numberof first and second trenches can also be produced simultaneously andparallel, corresponding to a corresponding number of structure elementsto be isolated or buried structures to be contacted.

A buried structure VS is preferably used with high-volt components andcan be present there at a depth d9 or d4 of circa 10 μm. The depth d8 ord5 of the first trench to be sufficient to isolate the buried structureelectrically with respect to adjacent buried structures, it may exceedthe depth d9 or d4 of the second trench by 50 percent, for example. Thewidth w2 of the second trench corresponds to at least twice the layerthickness of the dielectric layer DS plus a reserve of circa 1 μm, whichguarantees that the trench remains free. In this way, at least a cleartrench width of circa 1 μm remains after the deposition of thedielectric layer; it is suitable, after filling with conductivematerial, for the production of a sufficiently low-ohmic connection tothe buried structure. The width of the first trench is selectedcorrespondingly smaller than twice the layer thickness of the dielectriclayer, that is, for example, smaller than 2 μm with a layer thickness ofthe dielectric layer of circa 1 μm.

A trench is understood to mean round or rectangular recesses as well aslong-extended trench-like structures. In particular, the first trenchcan have an extension vertical to the represented plane of the drawings,which exceeds its width several times. In this way, large-surfacestructures can also be successfully isolated with such a trench, withrespect to adjacent structures.

The second trench can, on the other hand, have a round or squarecross-section, or also one formed differently, wherein, preferably, thelength and width of the trench opening do not differ too much. It isalso possible to contact a buried structure by means of several secondtrenches that are arranged next to one another.

The invention is suitable, in particular, for high-volt transistors thatrequire an increased electric isolation, which can be guaranteed withthe invention in a simple and low-cost manner.

1. A method for the production of a semiconductor component with a firstand a second trench in a substrate that comprises a semiconductor, inwhich the first trench is filled with a dielectric and in which thesecond trench is filled with a conductive material to form an electriccontact to a buried structure, wherein the method comprises the stepsof: (a) first, producing the two trenches in the semiconductorsubstrate, the first trench being produced with a width w1, and thesecond trench with a width w2, with w2>w1; (b) subsequently, depositinga dielectric over the entire surface and with edge covering, in such athickness that the first trench is completely filled with thedielectric, but so that the second trench still remains partiallyopened; (c) subsequently, etching the dielectric anisotropically untilthe substrate at the bottom of the second trench is freely exposed; and(d) subsequently, depositing an electrically conductive material overthe entire surface and with edge covering until the second trench isfilled.
 2. The method according to claim 1, wherein the first trench isproduced in step (a) with a depth d5 and the second trench with a depthd4, with d5>d4, so as to isolate the buried structure contacted by thefirst trench, with respect to the adjacent structure elements of thecomponents.
 3. The method according to claim 2, wherein the differentdepths of the two trenches are attained by a process in which, first,the first trench is etched anisotropically to a first depth d3, inwhich, subsequently, in a joint step, the first trench is further etchedanisotropically to its final depth d5 and, simultaneously, the secondtrench, to a depth d4.
 4. The method according to claim 3, wherein ahard mask of thickness d1 is placed on the substrate, which, after astructuring in the area of the first trench, has an opening with asubstrate surface exposed therein, and in the area of the second trench,a reduced layer thickness; wherein, subsequently, the trenches areproduced with an anisotropic etching process, which, for the substrate,has an etching speed that is substantially higher in comparison to thehard mask; and wherein the different depths of the trenches are adjustedby a suitable dimensioning of the layer thickness relative to theselectivity of the etching process.
 5. The method according to claim 1,(e) wherein before the trench etching, a first and, over it, a secondresist layer are produced and subsequently structured, wherein, in thefirst resist layer, an opening is produced for the first trench, and inthe second resist layer, openings for the first and second trenches areproduced; (f) wherein the substrate is etched anisotropically andselectively relative to the first and second resist layers, with thefirst trench being produced to a first depth d7; (g) wherein the firstresist layer is etched anisotropically relative to the second resistlayer, until the substrate is exposed in an opening of the first resistlayer provided for the second trench; and (h) wherein the substrate isetched anisotropically and selectively relative to the first resistlayer, with the first trench is being etched to a second and final depthd5, and the second trench to a depth d4.
 6. The method according toclaim 5, wherein for the etching of the substrates in steps (f) and (h),a chemically dominated plasma etching process is used.
 7. The methodaccording to claim 6, wherein during the ion etching, the pressureand/or gas composition of the plasma are changed, alternatingly, in sucha way that anisotropic etching and deposition of a passivation layeralternate with one another.
 8. The method according to claim 5, whereinsteps (g) and (h) are carried out in the same reactor.
 9. The methodaccording to claim 5, wherein the substrate is covered with a dielectricoxide/nitride double layer, into which, before step (e), openings havinga width that is greater than those of the corresponding openings in thefirst resist layer are etched with another resist mask in the area ofthe first and second trenches.
 10. The method according to claim 9,wherein the openings in the second resist layer are produced wider thanthe openings in the first resist layer.
 11. The method according toclaim 1, wherein for step (d), a silicide or doped silicon is depositedas a conductive material and, subsequently, the conductive layer,deposited over the entire surface, is back-etched or planarized untilthe conductive material is removed from all surfaces and remains only asa filling in the second trench.